1. Field of the Invention
The present invention relates to a vehicle and a method for controlling an engine which can be in a vehicle.
2. Background Art
Internal combustion engines can be required to operate at many different speeds, and under many different loading conditions. Although most engines include some type of cooling system, for example, a liquid cooling system utilizing a liquid-to-air heat exchanger such as a radiator, the engine may still become very hot during use. At such times, it is possible for a lubricating fluid, such as oil, to reach undesirably high temperatures. This can result in a loss of viscosity and oil pressure which may lead to inadequate lubrication of the engine components.
One attempt to deal with this situation is described in U.S. Pat. No. 5,070,832 issued to Hapka et al. on Dec. 10, 1991. Hapka et al. describes an engine protection system which derates engine performance as a function of fluid parameter fault conditions. Hapka et al. describes two derating schedules based on the level of the fluid parameter fault. In some cases, operation of the vehicle can be continued in a “limp home” mode. In other situations, the engine may be completely shut down.
One limitation of the engine protection system described in Hapka et al. is that once the derating schedules are implemented, the vehicle operator may not be able to operate the engine at a maximum engine speed. The ability to operate the engine at the maximum engine speed, even for a short period of time, may be important to the vehicle operator. Depending on the particular conditions the driver encounters, a short burst of speed may be necessary even when the temperature of the engine oil is above normal.
The issue of high oil temperatures may be particularly relevant to hybrid electric vehicles (HEV's), which may have a relatively small engine. The size of an engine in an HEV may be less than in a conventional vehicle, since many HEV's can combine the output torque of an electric motor with the torque of the engine to drive the vehicle. This allows the size of the engine to be reduced, thereby providing a cost savings and increased fuel economy. There may be times, however, when the motor cannot be used to augment the engine torque. In addition, even if the motor is used to augment the engine torque, certain driving situations—e.g., towing a heavy load, or traveling up a steep slope—may still impose significant loads on this relatively small engine.
Therefore, a need exists for a vehicle and a method for controlling an engine that does not allow the temperature of the engine oil to reach unacceptably high levels, yet at the same time, allows the vehicle operator to operate the engine at the maximum engine speed, for at least a short period of time, under certain conditions.